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How modern methods of radiosurgery transform treatments for skull base brain tumours

Innovative developments in imaging and technology have transformed the way skull base brain tumours are treated and how patients recover. Patrick Grover, consultant neurosurgeon at University College Hospital London and Amethyst Radiotherapy UK, shares his take on the developments of these treatments and the importance of the multidisciplinary team.

Although the majority of skull base brain tumours are benign and slow growing, they can be complex to treat and present a risk to brain function. The bones that form the skull base also form the eye socket, nasal cavity roof, part of the sinuses and the bones surrounding the inner ear. An abnormal growth here can put specific brain functions at risk, and so often warrant treatment.

As these tumours form deep within the skull base, surgical excision can be difficult and requires a specialised team. There can be a risk of damage to the nerves and blood vessels surrounding the tumour, in some cases making surgery unfeasible. Treatments for brain tumours are regularly advancing as developments in imaging and technology are integrated into modern medical practice. Focal radiation treatments such as Gamma Knife radiosurgery have transformed the management of these tumours over recent decades.

Radiation can be delivered in different forms. The most focused and targeted is termed stereotactic radiosurgery. For well circumscribed, deep-seated tumours, this is the best option as this form of treatment can target a high dose of radiation to the tumour, whilst minimising any spread to the ‘normal’ brain. This is because it uses up to 192 focused beams of gamma radiation, which individually carry a low dosage and converge at a single point to deliver a high dosage of radiation, targeting the intended area only.

Gamma Knife radiosurgery: non-invasive focal therapy

One of the most common forms of stereotactic radiosurgery is Gamma Knife radiosurgery. This technique achieves the highest level of accuracy by using a stereotactic frame that is fitted to the head during the treatment. The other advantage of radiosurgery is that it is non-invasive and therefore, in some circumstances, it avoids the need for open surgery. It is convenient and performed as a day case.

However, it does have some limitations and cannot be used to remove a tumour that is already causing symptoms. The aim is to arrest growth, and, as a general rule of thumb, a tumour that is less than 3cm in size can be considered for radiosurgery. Smaller is better for this treatment type, as larger tumours can be susceptible to swelling reactions in the brain. Larger or more diffuse tumours may be considered more suitable for conventional fractionated radiotherapy instead over a number of sessions of treatment.

The majority of tumours, particularly those of a smaller size, are suitable for treatment with stereotactic radiosurgery techniques such as Gamma Knife radiosurgery. As this has the benefit of being non-invasive, it has a lower risk of injury – for example, to the facial nerve, in the region of 1% of cases or less.

Gamma Knife radiosurgery does not remove the tumour, but it can help prevent further growth in approximately 95% of tumours, or more, depending on the size. There are risks of exacerbating balance problems, for example, for a period of time, or accelerating hearing loss, but compared with ongoing tumour growth these side effects are usually well tolerated. Treatment is carried out as a day case, and normal activities including work can continue within a few days.

Treating large skull base tumours

For large tumours, especially those causing significant pressure on the brain resulting in symptoms, surgical removal performed by a highly experienced team is the preferred treatment. Depending on the location and critical structures involved, this may be a total or partial removal. With a total excision, the chance of recurrence in the future is extremely low. The pressure on the brain should be relieved with the prevention of further symptoms, and there is often improvement in existing symptoms.

This improvement in symptoms may take place over a period of months as the swelling gradually settles with time. If there is any residual tumour, this is checked with serial imaging, and any further growth then typically treated with radiation therapies.

What are the symptoms of skull base tumours?

The symptoms appear slowly as the tumours grow and specific symptoms will depend on the type, location and size of the tumour. Common symptoms include:

  • Weakness
  • Hearing loss
  • Numbness
  • Vision loss
  • Double vision
  • Problems with balance.

A subgroup of these tumours can sometimes be diagnosed incidentally when scans are performed for other reasons.

The ideal form of imaging for an accurate diagnosis is an MRI of the head with contrast medium. This will show the characteristics of the tumour, and its relationship with the surrounding brain structures. Once a diagnosis is made, a treatment plan can then start to be formulated.

In some cases, the scan may reveal that the tumour is causing swelling, or oedema, in parts of the brain. This oedema can cause symptoms in addition to those that result purely from the pressure of the tumour itself. If the oedema is significant and causing symptoms, treatment with steroid medication, such as dexamethasone, may be needed for a period of time as decided by the clinician. However, because long-term use of steroids is associated with medical complications such as weight gain and osteoporosis, their use is usually restricted to short periods.

Skull base tumours treated with Gamma Knife

Vestibular schwannoma and meningioma are both examples of benign skull base tumours that have similar behaviour and treatment options where complex management approaches are often required, including Gamma Knife radiosurgery.

Vestibular schwannoma

Vestibular schwannoma (also referred to as acoustic neuroma, acoustic neurinoma, or acoustic neurilemoma) are formed by the overproduction of the Schwann cells that support and insulate the vestibular and cochlear nerves. As such, these most commonly present with balance issues, hearing loss or tinnitus on one side. As they grow larger, these tumours can press on the trigeminal nerve and cause facial numbness and, in some cases, shooting pains.

These tumours grow approximately 1- 2 mm a year and are primarily treated with either surgical removal or focused radiation treatments. The location of these tumours significantly affects the decision on what treatment is best. They grow in a confined space between the hearing structures of the ear and the brainstem, called the cerebellopontine angle.

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There is very little space here to accommodate tumour growth or swelling, and there are many critical nerves and blood vessels. The brainstem itself is densely packed with fundamental neural structures. In this context, the management of these tumours should be guided by an experienced multidisciplinary team with the ability to provide all types of treatment to a very high level.

The main difficulty with treating vestibular schwannoma is the proximity of the nerve to the face on that side. This facial nerve is immediately adjacent to the hearing and balance nerve, and because of this it becomes progressively stretched by tumour growth. This stretching happens slowly, which means that the nerve is rarely impacted, and, in fact, can be flattened very significantly by the tumour without any loss of function. However, because it is compressed, it is susceptible to injury from treatments for the tumour, resulting in facial weakness or, in some cases, complete paralysis.

In the first instance, it is usual best practice to monitor the tumour and assess its behaviour with a repeat scan within six months. A significant proportion of tumours, particularly those of a smaller size, do not grow, and these may be followed up long term with an MRI scan once a year. If the tumour is stable for a number of years, this interval can be increased.

Treating large vestibular schwannoma tumours

Large tumours causing swelling in the ‘normal’ brain usually require surgical intervention.  If it is of a very large size, up to 4cm, for example, the tumour may prevent normal brain fluid exiting the brain, and this is called hydrocephalus. When this happens, a ventriculoperitoneal shunt can be inserted in order to bypass the obstruction, and this carries cerebrospinal fluid from the brain to the abdomen, where it is absorbed.

For tumours above 2.5cm, surgery is usually considered in order to remove as much of the tumour as possible and relieve the pressure on the brain. Such surgery is usually performed in specialist centres with sufficient expertise in the management of such tumours. Due to the technical demands of this treatment, these operations are usually performed collaboratively with ENT and neurosurgery skull base surgeons.

Depending on the configuration of the tumour, and surrounding structures, this may be most commonly performed via a translabyrinthine approach. This method traverses the hearing and balance structures of the ear. Alternatively, a retrosigmoid approach may be performed alongside the brain’s balance organ, the cerebellum. There is also the option of middle fossa approaches in selected circumstances.

The crucial aspect of surgery is to safely remove enough tumour without damaging normal structures, such as the facial nerve. During surgery this nerve is monitored with an electrophysiological probe to ensure it is functioning normally. It is our practice to perform facial nerve sparing surgery where a portion of tumour is often deliberately left behind on the nerve to preserve its function. If this residual tumour grows subsequently, it can then be safely treated with radiosurgery in the future.

Meningiomas

Meningiomas are benign tumours that grow from the lining of the brain known as the meninges. They grow slowly, also usually around 1-2 mm a year, and do not spread to other parts of the body. However, depending on where they grow, they can start to cause symptoms by pressing on the normal structures of the brain.

In regions of the brain where there aren’t many important functions, they can grow very large before symptoms are experienced and diagnoses are made. This is particularly true of tumours that press on the frontal lobes of the brain, as meningiomas most commonly form in the anterior cranial fossa, and these cause more subtle problems, such as personality changes or headaches. When located further back on the posterior cranial fossa, a tumour can cause a range of symptoms.

Treatment options for meningiomas

The three main options for treating meningiomas are:

  • Conservative management with ongoing imaging
  • Radiation treatments, including Gamma Knife radiosurgery
  • Surgical removal or cytoreduction known as debulking.

The majority of tumours can be observed in the first instance with surveillance imaging. This period is important to determine the behaviour of the tumour, and a six-monthly MRI scan is often appropriate. If new symptoms develop in the meantime, the MRI scan can be brought forward. The observational period allows time to fully characterise the tumour diagnosis so that the most appropriate treatment options are considered and decided upon. In these circumstances, if tumour growth is demonstrated at six months, a clear and quick decision on the best form of treatment can be made.

Conclusion

Gamma Knife treatment has a long, proven history of success in treating skull base tumours, and it is widely considered a ‘gold standard’ of care for dealing with both benign and malignant brain tumours. It has been used as a treatment modality for small-medium sized vestibular schwannoma since the 1990s and has since become the first-line treatment approach.

As an illustration, a 2014 research study from the Journal of Neurosurgery, assessed the long-term clinical results following Gamma Knife treatment for a large cohort of vestibular schwannomas. It found that control of the tumour with Gamma Knife radiosurgery was achieved in 97.1% of the patients. In 82.7% of the patients, the tumour volume had decreased at the last follow-up, with a mean relative reduction of 34.1%.

The rate of complications was very low, with most consisting of a transient worsening of pre-existing symptoms. Patients who had vertigo, balance disorders or facial or trigeminal impairment usually experienced complete, or at least significant, symptom relief after treatment.

In summary, the treatment of skull base tumours such as vestibular schwannomas and meningiomas is complex with many parallels between the two. Each patient and tumour must be assessed individually, and the range of possible treatment options presented to allow an informed decision to be made for the particular circumstances.

It’s important for patients to see an experienced multidisciplinary team that can provide all the available options so that treatment can be tailored appropriately. 

Mr Patrick Grover is a renowned consultant neurosurgeon, practising privately for Amethyst Radiotherapy’s London Clinic at the Queen Square (Gamma Knife) Radiosurgery Centre, while also serving as the cranial neurosurgery lead and consultant skull base vascular neurosurgeon at the National Hospital for Neurology and Neurosurgery at University College London Hospitals NHS Foundation Trust.

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